Using SM Version 15.5.6222.
According with the documentation (http://help.smartertools.com/SmarterMail/v15/Default.aspx?p=_SA&v=15.5.6222&lang=en-US&page=systemadmin/frmprotocolsettings) if the item "Append authenticated as header for outgoing messages" in SMTP OUT is checked the header "x-smartermail-authenticatedas" exists in all outbound messages, but, in version 15.5.6222 don't work.
In others version work (such 14.x) but in 15.5, don't.